The scaffold attachment factor/nuclear matrix protein SAFB maps to a locus with extremely high loss of heterozygosity in human breast cancer. Its expression is also reduced in many breast cancers, and in a xenograft model of antiestrogen resistance. Recently we discovered that SAFB binds directly to the estrogen receptor (ER), and functions as an ER corepressor. SAFB binding to ER is increased by the antiestrogen tamoxifen, and SAFB can enhance tamoxifen-mediated repression of ER. The repressive effect of SAFB on ER activity appears to involve chromatin remodeling, since repression is relieved by the histone deacetylase (HDAC) inhibitor trichostatin A. Supporting this, we have recently mapped an independent repression domain in SAFB that binds proteins with LIM and RING domains which have been shown to be involved in chromatin-mediated regulation of gene expression.We therefore propose to define the function of SAFB in regulating ER function, making use of our unique model systems that will elucidate specific SAFB effects upon ER in vitro and in vivo, and the role of loss of SAFB in the onset of antiestrogens resistance. Specifically, we will ask: 1) Which classes of endogenous estrogen-regulated genes are affected by SAFB, and how does this translate to growth effects? We will investigate the growth and gene expression effects of inducible changes in SAFB levels in a unique model, comparing ER-negative MCF-7-derived cells (C4-12) vs. C4-12 cells with restored functional ER (C4-I2ER-HA). 2) Does SAFB-mediated repression of ER involve chromatin remodeling? Because of the evidence implicating chromatin remodeling in the ER-repressive effects of SAFB, we will investigate the involvement of interaction with HDACs and other proteins implicated in modulating chromatin structure. 3) Is SAFB expression critical for the development of estrogen-responsive tissues, and is this reflected by altered expression of estrogen target genes in vivo? We will address these issues in SAFB-null mice. 4) Is SAFB involved in the development of antiestrogen resistance in breast cancer cells? Because SAFB enhances the ability of antiestrogens like tamoxifen to inhibit ER-mediated transcription, and SAFB is reduced in a xenograft model of tamoxifen resistance, we will test directly whether decreased SAFB levels change expression of estrogen-regulated genes in this model and enhance the onset of resistance.